Molecular and genetic advancements in understanding cancer cell biology have revealed genes and signaling pathways that regulate proliferation, metastatic potential and chemotherapeutic resistance of cancers. This has led to the development of relatively low-toxicity targeted chemotherapies employed in personalized treatment to interrupt signaling pathways expressed by a patient's cancer. Resistance to targeted chemotherapies occurs due to alternate pathways and downstream effects. Transcriptional analysis of resistant tumors may identify additional targeted therapies that can be added to maintain tumor control. Inherent in the success of this approach is the ability to identify early signs of response vs. resistance to single agent targeted chemotherapy. As these drugs are often cytostatic, conventional anatomic imaging to identify early response is typically inadequate. Our preliminary data suggests that FDG PET/CT alone may be inadequate in identifying early treatment failures in certain therapeutic clinical trials. We hypothesize that a multi-tracer molecular imaging profile that employs assessments of metabolic rate (18F-FDG), cellular proliferation [18F-fluorothymidine (FLT)] and blood flow (15O-H2O) will provide an early and more accurate assessment of response vs. resistance to targeted chemotherapies than either single agent PET or anatomic measurements. This imaging response profile may differ as a function of the type of tumors, its genetic or molecular signatures, and the type of targeted chemotherapy employed. We further posit that an imaging approach that requires three separate PET scans for each point in therapeutic assessment and the use of short-lived 15O-H2O to assess blood flow will never be broadly adopted in clinical practice. To overcome this, we hypothesize that the early uptake phase of 18F-FDG or 18F-FLT PET can be used to characterize blood flow, substituting for 15O-H2O. We also hypothesize that serial injections of FDG and FLT with a single dynamic scan can be separated to recover the relevant response measures for each tracer. The specific aims of this project are: Aim 1: To develop a molecular imaging profile of early response and resistance in patients with advanced cancer enrolled in early phase therapeutic trials testing the efficacy of novel single agent targeted chemotherapies. Both at baseline (before treatment) and at 28 days (+/- 4) into treatment, three separate PET/CT scans will be performed utilizing 18F- FDG (metabolism), 18F-FLT (proliferation), and 15O-H2O (blood flow). The initial molecular imaging profile and the changes that occur during the first month of therapy will be compared to anatomic RECIST measurements, molecular signatures, and patient outcome. Aim 2: To extend and apply rapid (single-scan) multi-tracer PET tumor imaging techniques to dual-tracer FDG+FLT imaging of early response and resistance in this patient population, evaluating the feasibility and accuracy of these methods using the clinical scan data acquired under Aim 1. The clinical data will also be used to further validate techniques being developed in our laboratory for characterizing tumor blood flow from the uptake phase of FDG and FLT, using 15O-H2O as a gold standard.